It seems like mail sent to me gets stuck in the deferred que. What happens according to the log file is that the av software receives it then tries to resend it to my external IP (im behind a NAT firewall) It then times out.
zimbra.log:
status=deferred (connect to xxxx.biz[xx.xxx.xx.xx]: Connection timed out)
(private info xx'ed out)

My problems started after i checked "Enable DNS lookups". Before i did that i couldn't send mail out, but received just fine.

I tried following this "http://www.tickus.com/?q=node/15" (i use ubuntu 6.10 ) to setup BIND, but host xxxx.biz still resolves to my external IP even though all other nslookup or ping i do resolves to my internal IP.

As it tries to send mail to your public IP address then your DNS isn't correct. Check the forums and wiki for some diagnostic steps.

First of all I don't understand why it should matter if it sends via internal or external IP. I can telnet (and thus send mail) from my zimbra installation to my external IP, so why can't zimbra do the same?

Secondly, I have looked both in the wiki, the forums and googled for answers to the DNS and everything EXCEPT host xxxx.biz works like it should. If you have a link to somwhere with diagnostics steps for this it would be appreciated.

Because Zimbra relies on DNS lookup (since you've activated the feature) while telneting to the MTA does not...

Take a look at the wiki, about "Split DNS" : you need to setup bind inside (on your Zimbra server or on another server) with your Zimbra server defined as MX for your domain (and obviously tell your Zimbra server to use this bind as resolver).

First of all I don't understand why it should matter if it sends via internal or external IP. I can telnet (and thus send mail) from my zimbra installation to my external IP, so why can't zimbra do the same?

It matters because postfix uses DNS to lookup an address, if you're behind a NAT router (and it doesn't support loopback, most dont) and you try to send mail to Zimbra, if it then gets your public IP it doesn't know what to do with it - it basically says 'I'm already here' and because you're _behind_ the NAT device then no port forwarding takes place.

Originally Posted by devzero

Secondly, I have looked both in the wiki, the forums and googled for answers to the DNS and everything EXCEPT host xxxx.biz works like it should. If you have a link to somewhere with diagnostics steps for this it would be appreciated.

Using the command 'dig yourdomain.com any' (or MX) will tell you whether your host is resolvable or not and where it thinks it's located, you can also try 'host `hostname`'.